Field of the Invention
The present invention relates to a numerical controller, and particularly relates to a numerical controller capable of suppressing a backlash that occurs during tool tip point control.
Description of the Related Art
Tool tip point control is control that drives a control point of a tool so that a tip point of the tool moves on an instructed movement path at a +n instructed speed even under an instruction that causes the posture of the tool to change from moment to moment in five-axis machining in which movement of linear axes and movement of rotational axes are simultaneously instructed.
Regarding such tool tip point control, Japanese Patent Laid-Open No. 2003-195917 discloses a numerical controller for controlling a five-axis machine tool, the numerical controller being capable of linearly driving a table along an X-axis and a Y-axis while rotating the table about a C-axis, which is a rotation axis about a Z-axis perpendicular to the X-axis and the Y-axis, and linearly driving a tool along the Z-axis while rotating the tool about an A-axis, which is a rotation axis about the X-axis.
In the tool tip point control of the numerical controller disclosed in Japanese Patent Laid-Open No. 2003-195917, an instruction for moving a tool tip point is converted to an instruction for moving a tool control point once, and, in the middle of the path, the tool control point is driven so that the tool tip point moves on an instructed movement path. For example, as shown in FIG. 12, when an instruction for moving the tool tip point is given in tool tip point control mode, the posture of a tool changes moment to moment but the tool tip point is controlled so as to move on an instructed straight line. In this case, the path of the control point is responsive to the change of the posture of the tool and thus forms a curved line as shown in FIG. 12.
As described above, during tool tip point control, a path of a control point generally forms a curved line. This causes a problem in that the path of the control point easily reverts (makes a backlash) in the middle of a block during tool tip point control. For example, under the instruction illustrated in FIG. 13A, the control point path appears to form a straight line. Actually, however, the control point path slightly reverts in a Y-axis direction in each instruction block. Specifically, as shown, the control point path reverts in the Y-axis direction at a point A, which is in the middle of a block, and also at a point B, which is a joint portion between blocks.
Accordingly, during tool tip point control, the path easily reverts, invoking a compensation function such as backlash correction and backlash acceleration. Such compensation function, however, tends to be overly effective, causing a problem of adversely affecting a machined surface quality.